Montgomery County officials face a tough choice if they want to build a rapid bus system: spend as much as $2.5 billion on new lanes or run the buses on already clogged county roads.

The system can be built for $578 million, excluding the costs of buses or stations, if the county does not buy any additional land to expand the roads, a new study found. The finding offers an alternative to the $2.5 billion price tag consultant Parsons Brinckerhoff gave last year when it studied a larger-scale system.